Apparatus for the measurement or machining of an object, provided with a displacement stage with wedge-shaped guides

ABSTRACT

An apparatus provided with wedge-shaped elements is proposed for the precision positioning of objects  37  to be measured or machined. A wedge-shaped translational body  32  is carried by two bearing wedges  24, 26 . In accordance with the invention the wedges can be independently translated in the x direction. The body is translated in the z direction by moving the bearing wedges over an equal distance but in opposite directions. The body is translated in the y direction by moving the bearing wedges over an equal distance in the same direction. The mutually moving parts of the apparatus are carried by means of fluid bearings. A high precision is thus obtained, because fewer errors due to non-infinite stiffnesses and error motions are accumulated. A laser measuring apparatus  74, 38, 40  is used for measuring and adjusting the position of the object carrier  35.

The invention relates to an apparatus for the measurement or machiningof an object, which apparatus is provided with a displacement stage forthe translatory displacement in at least one co-ordinate direction of apart of the apparatus which is to be displaced relative to a frame ofthe apparatus, which displacement stage includes:

two supporting wedge-shaped displacement guides which bear on asupporting surface of the apparatus and each of which is provided with awedge face, said guides being arranged opposite one another in such amanner that their wedge faces face one another,

a displacement member which is connected to the part of the apparatus tobe displaced and is provided with two wedge faces which co-operate withthe wedge faces of the displacement guides, the displacement memberbeing supported by the wedge faces of the displacement guides as aresult of said co-operation,

which apparatus is also provided with drive means for the displacementof the two displacement guides in such a manner that the distancebetween their wedge faces varies while the orientation of said wedgefaces relative to one another remains the same.

Apparatus of this kind can be used, for example, in component-mountingmachines, in wafer steppers for the manufacture of integrated circuits,in printers or in co-ordinate measuring machines for determining theshape of an object to be measured. The part of the apparatus which is tobe displaced relative to a frame of the apparatus will generally be anobject carrier for an object to be machined or measured by means of theapparatus. However, it is alternatively possible for this object tooccupy a fixed position relative to the apparatus during the measurementor machining operation; in that case the part of the apparatus to bedisplaced is a measuring probe or a machining tool such as, for example,a cutter.

A displacement stage for use in an apparatus of this kind is known from“Motion Control 1997 Product Catalog”, issued by “Micro-Controle, Memberof the Newport Group”. The displacement stages described in the citeddocument, notably on the pages 2 to 38, are referred to therein as “UZMSeries Motorized Vertical Translation Stages”. The two wedge faces ofthe two supporting displacement guides in this known displacement stagetogether form a trough having a V-shaped cross-section. The displacementmember is arranged in said trough while the wedge faces of thedisplacement member bear on the wedge faces of the displacement guides.The displacement member is thus supported by the wedge faces of thedisplacement guides. In order to displace the displacement member in oneco-ordinate direction in this known displacement stage (for example, inthe vertical direction), the displacement guides are displaced relativeto one another in the horizontal direction by means of drive means inthe form of a lead screw. Because the displacement of the twodisplacement guides is such that the distance between their wedge facesvaries while the orientation of these wedge faces relative to oneanother remains the same, the V-shaped trough becomes wider or narrowerwithout the orientation of the trough being changed. One way to preventthe displacement member from undergoing also an undesirable displacementin a direction transversely of the desired direction, one displacementguide should be subject to a displacement which is equal to but opposedto that of the other displacement guide. Another way is to guide theassembly formed by the displacement guides and the displacement memberin a housing.

In the technical field concerning apparatus for the measurement ormachining of an object, notably apparatus of this kind for precisiondisplacement, there is often a need for a controlled displacement inmore than one co-ordinate direction. When the known displacement stageis used, the desired effect can be achieved by stacking a plurality ofstages in such a manner that each stage of the stack provides adisplacement in a respective co-ordinate direction transversely of theother two. Thus, an x displacement, a y displacement and a zdisplacement can be realized by using three stages, it being possible toimpart an individual magnitude to each of the three displacements, thatis, independently of that of each of the other displacements.

However, said stacking of the displacement stages results in aconstruction in which the positional uncertainty of a part of theapparatus supported by such a stack increases. Such positionaluncertainty is caused inter alia by an accumulation of positionaluncertainties which are due to the non-infinite stiffness of each of thedisplacement stages of the stack, and by an accumulation of thegeometrical deviations of the components in the displacement stages.Moreover, the dimensions of such a stack are comparatively large and itis also necessary to displace drive means such as motors, so that forcesarise due to supply cables and additional mass must be displaced.

It is an object of the invention to provide an apparatus of the kind setforth in which the cumulation of positional uncertainties due tonon-infinite stiffnesses is strongly reduced and a comparatively compactconstruction is possible.

To this end, the apparatus in accordance with the invention ischaracterized in that it is provided with control means for controllingthe drive means, and that the drive means and the control means arearranged for mutually independent displacement of the two displacementguides. Because the displacement guides are displaceable independently,a mutually equal but opposed displacement can be imparted thereto, likein the known displacement stage. Consequently, the V-shaped troughformed by the displacement member becomes wider or narrower, however,without the center of the trough being displaced. Consequently, thedisplacement member resting in the trough moves in known manner, thatis, perpendicularly to the displacement direction of the displacementguides, for example, in the z direction. However, utilizing theindependent motion in accordance with the invention, it is also possibleto impart a mutually unequal and opposed displacement to thedisplacement guides. For example, the same displacement in the samedirection can be imparted to the guides; in that case the displacementmember moves in a direction perpendicular to the former direction, forexample, the x direction. Using an arbitrarily chosen combination ofdisplacements of the displacement guides, any desired displacement canthus be realized in the z direction as well as the x direction, withoutstacking of displacement guides and hence without the associatedaccumulation of positional uncertainties.

In a preferred embodiment of the invention the wedge faces of thedisplacement member bear on the wedge faces of the displacement guidesby way of fluid bearings. Bearings of this kind, constructed mainly asair bearings, are generally known. It is a known property of suchbearings that their bearing stiffness can be made very high. Anotherknown property is that they exhibit only minor rotational and/ortranslational error motions. Their use in a precision apparatus inaccordance with the invention offers the advantage that the positionaluncertainty of the part of the apparatus to be displaced can besignificantly reduced by the non-infinite stiffness of the bearing ofthe displacement member on the displacement guides, and that only minorerrors occur in the guiding of the motion.

In another embodiment of the invention the displacement guides bear onthe supporting surface of the apparatus by way of fluid bearings. Thisagain offers the advantage that the positional uncertainty of the partof the apparatus to be displaced can be significantly reduced by thenon-infinite stiffness of the bearing of the displacement guides on thesupporting surface of the apparatus, and that only minor errors occur inthe guiding of the motion.

In another embodiment yet of the invention the displacement guides aredisplaceable across the supporting surface of the apparatus along linearguides which are provided on the supporting surface, the displacementguides bearing on the linear guides by way of fluid bearings. This againoffers the advantage that the positional uncertainty of the part of theapparatus to be displaced can be significantly reduced because of thenon-infinite stiffness of the bearing of the displacement guides on thelinear guides, and that only minor errors occur in the guiding of themotion.

In another embodiment yet of the invention the displacement member isdisplaceable, relative to the displacement guides, in the direction ofthe line of intersection of its wedge faces, the apparatus beingprovided with a drive member which contacts the displacement member inorder to drive said displacement, the contact between the displacementmember and the drive member being realized by way of a fluid bearing.Said displaceability of the displacement member in the direction of theline of intersection can be readily achieved by way of the fluid bearingbetween the wedge faces of the displacement member and the displacementguides. This is because a bearing of this kind enables mutualdisplacement of the two faces in two mutually perpendicular directions.In the case of a fixed attachment of the drive member to thedisplacement member, the drive member for driving the displacementmember would have to move along in directions transversely of its drivedirection, thus giving rise to undesirable forces between the twocomponents. Using a fluid bearing between the two components it isachieved that the drive member can exert a force exclusively in thedirection of said line of intersection, irrespective of the lateralposition of the displacement member.

The part of the apparatus to be displaced in a further embodiment of theinvention is provided with at least one mirror surface, the displacementin at least one co-ordinate direction being measured by means of a laserdistance sensor which comprises said mirror surface. For the precisionof the determination of the position of the object to be measured ormachined it is important to avoid sources of positional uncertainty asmuch as possible. This means that it is advantageous to determine theposition of the part of the apparatus to be displaced itself instead of,for example, that of the displacement guides. Therefore, the mirrorwhich forms part of the laser distance sensor is provided on the part ofthe apparatus to be displaced itself. When use is made of a precisionapparatus, not only the required precision is of importance, but alsothe maximum dimensions of the object to be measured, so the measuringrange of the apparatus. In conformity with the invention, a measuringrange of 100 mm can be realized quite well. For suitable measurement ofthe positional precision to be achieved by the part of the apparatus tobe displaced in the case of such a measuring range (that is, to avoidthe introduction of substantial additional uncertainties by themeasurement itself), a laser distance sensor is chosen for thedetermination of the displacement.

The invention will be described in detail hereinafter with reference tothe Figures in which corresponding elements are denoted by correspondingreference numerals. Therein:

FIG. 1 illustrates diagrammatically the principle of a displacementstage according to the state of the art;

FIG. 2 a shows diagrammatically the construction of a measuring ormachining apparatus in accordance with the invention;

FIG. 2 b is a side elevation of a wedge-shaped displacement guide withits linear guide in accordance with the invention;

FIG. 2 c is a plan view of a wedge-shaped displacement guide inaccordance with the invention;

FIG. 2 d is a perspective view of the displacement member in accordancewith the invention;

FIG. 2 e is a diagrammatic representation of the fluid bearing for the ydrive;

FIG. 3 a is a first perspective view of the apparatus in accordance withthe invention, and

FIG. 3 b is a second perspective view of the apparatus in accordancewith the invention.

FIG. 1 shows an apparatus for the measurement or machining of an objectas generally known from the state of the art. The apparatus includes aframe 2 in the form of a base plate. It is assumed that the apparatusshown in FIG. 1 is arranged for the measurement of an object. The objectto be measured can be positioned on the base plate 2, the shape and/orthe dimension of the object being determined by scanning by means of ameasuring probe 4. Generally speaking, the object can be displacedrelative to the measuring probe which is rigidly arranged relative tothe apparatus, or the measuring probe can be displaced relative to theobject which is rigidly arranged relative to the apparatus. FIG. 1refers to the latter situation. The part of the apparatus to bedisplaced relative to the frame 2 of the apparatus, therefore, is themeasuring probe 4 in the present case. The measuring probe isdisplaceable in three mutually perpendicular co-ordinate directions x, yand z, said displacements being realized by an assembly of threemutually independent displacement stages 6, 8 and 10 for displacement inthe x direction, the y direction and the z direction. In order todetermine the position of the measuring probe, there are provided threeoptical rulers 14, 16 and 18 for the x direction, the y direction andthe z direction, respectively. The measuring probe 4 is attached to adisplacement member 12. In order to realize the desired displacements ofthe respective displacement stages, there may be provided (not shown)drive means for the displacement of the stages, for example, linearmotors. The position of the measuring probe 4 is determined by the sumof the positions of the individual stages 6, 8 and 10. Because of thisaddition of the positions of the stages, however, the positionaluncertainty of the measuring probe 4 is also formed by the sum of thepositional uncertainties which are due to the non-infinite stiffness ofeach of the stages 6, 8 and 10 of the assembly, and by addition of thegeometrical errors of the components n each of the stages. Moreover,such an assembly has comparatively large dimensions as is illustrated,for example, by the dimensions of the z stage (and note that thedisplacement range of this stage must still be added thereto).

FIG. 2 is a diagrammatic representation of relevant elements of thedisplacement stage in accordance with the invention. FIG. 2 a is adiagrammatic front view of the apparatus in accordance, with theinvention with the displacement stage and the measuring probe. Theapparatus as shown in FIG. 2 a includes a frame 2 which comprises asupporting surface 20 and a carrier 22 for the measuring probe 4. On thesupporting surface 20 there are arranged two wedge-shaped supportingdisplacement guides 24 and 26, each of which is provided with a wedgeface 28 and 30, respectively, in such a manner that these wedge facesare oppositely arranged so as to face one another. A displacement member32 is arranged between the displacement guides 24 and 26; this member isprovided with two wedge faces 34 and 36 which co-operate with the wedgefaces 28 and 30, respectively, of the displacement guides 24 and 26,respectively. This co-operation ensures that the displacement member 32is supported by the wedge faces 28 and 30 of the displacement guides 24and 26.

On the displacement member 32 there is mounted an object carrier 35 forcarrying an object 37 to be measured by means of the apparatus. Theobject carrier 35 is configured as three mutually perpendicularlyextending surfaces, each of the outer sides of which is provided with amirror for determining the position of the object carrier 35 by means ofa laser distance sensor (not shown in the Figure); only two of saidthree mirrors are shown, that is, the mirrors 38 and 40. Thedisplacement member 32 is provided with a comparatively large opening 42for the passage of the laser beam which measures the z position. Each ofthe displacement guides 24 and 26 is also provided with an opening 44and 46, respectively, for the passage of said laser beam; these openingshave comparatively small dimensions as will be described in detail withreference to FIG. 2 c. The assembly formed by the displacement guides 24and 26 and the displacement member 32 and the supporting surface 20constitutes the displacement stage 48. In the present embodiment thepart of the apparatus to be displaced is formed by the combination ofthe displacement member 32 and the object carrier 35.

For the displacement of the displacement guides 24 and 26 there areprovided drive means which are not further specified and arediagrammatically represented by arrows 50 and 52 in the Figure. Suchdrive means may have any appropriate form, for example, a linearelectric motor or a hydraulic drive. The drive means 50 and 52 aremutually independent, i.e. the displacement in the x direction of thedisplacement guide 24 can take place irrespective of the position of thedisplacement guide 26 and vice versa. Because of the choice of the wedgeshape for the displacement guides 24 and 26, the distance between theirwedge faces 28 and 30 can vary during their displacement across thesupporting surface 20 in the x direction while the orientation of saidwedge faces relative to one another remains the same. The drive means 50and 52 can be controlled in any suitable way; preferably, driving takesplace under the control of a suitably programmed computer 54.

The displacement guides 24 and 26 are journaled on the supportingsurface 20 by means of known fluid bearings. The displacement member 32bears on the wedge surfaces 28 and 30 also by means of fluid bearings.Bearings of this kind will be described in brief with reference to FIG.2 d.

When the displacement guides 24 and 26 are driven in accordance with theinvention, a displacement of the displacement member 32 can be realizedin the z direction as well as in the x direction. For the formerdisplacement the driving of the displacement guides 24 and 26 iscontrolled in such a manner that they perform an equally large butopposed displacement, for example, towards one another. As a result, thedisplacement member 32 is displaced in the positive z direction. For thelatter displacement the driving of the displacement guides 24 and 26 iscontrolled in such a manner that they perform an equally largedisplacement in the same direction, for example, in the positive xdirection. As a result, the displacement member 32 is displaced in thepositive x direction. Evidently, any combination of the former and thelatter displacement is feasible.

FIG. 2 b is a side elevation of one of the displacement guides, forexample, the guide 26. This Figure shows how the displacement of thiscomponent is realized in the x direction. A linear guide 56, having arectangular cross-section, is provided on the supporting surface 20 anda corresponding recess is provided in the displacement guide 26. Thedisplacement guide bears on the linear guide by way of fluid bearings.

FIG. 2 c is a plan view of one of the displacement guides, for example,the guide 26. This Figure shows the opening 46 for the passage of thelaser beam of the distance sensor. It is to be noted that the use of theconfiguration in accordance with the invention makes it possible toutilize comparatively small openings, so that a displacement of thedisplacement member 32 in the y direction is possible by way of saidfluid bearings, that is, without a y displacement of the guides 24 and26 being required for this purpose. This is because this displacementcomponent is realized by a y displacement of the displacement member 32.The latter member can be provided with a comparatively large opening 42for the laser beam, without the supporting of this member by way offluid bearings being impeded. This is because such bearing takes placeon the wedge faces 34 and 36.

FIG. 2 d is a more detailed perspective view of the displacement member32 in which the wedge face 36 with the elements for the fluid bearing onthe corresponding wedge face 30 of the displacement guide 26 (FIG. 2 a)are visible. The fluid bearing comprises four bearing areas 58 a to 58d, each of which is provided with a number of feed holes 60 a to 60 dfor feeding the fluid (for example air) under pressure to the bearinggap. The surface of the bearing area is defined by a groove (forexample, 62) around this area. Furthermore, the wedge face 36 isprovided with a preload area 64 in which an underpressure is created viaoutlet holes which are provided in a groove 66 extending around thepreload area 64. The surface of the preload area 64 is defined in knownmanner by the combination of the grooves 62, the groove 66 and theintermediate area. The forces acting on the wedge face 36 due to thefluid bearing (that is, four forces exerted in the direction towards thewedge face by the four bearing areas 58 and one force exerted in thedirection away from the wedge face by the preload area 64) willgenerally cause a turning moment on the displacement member 32; such aturning moment has an adverse effect on the precision of the positioningof this member. Because of the symmetrical wedge shape of this member,the advantage is achieved that such moments cancel one another.

FIG. 2 e is a diagrammatic representation of the fluid bearing for the ydrive of the displacement member 32. The right-hand part of this Figureis a view in the same direction as in FIG. 2 a, that is, a front face ofthe displacement member 32 with the wedge faces 34 and 36 extendingperpendicularly to the plane of drawing. The left-hand part of FIG. 2shows a surface 68 which co-operates with said front face and isprovided with the elements necessary for a fluid bearing. Such a fluidbearing has already been described with reference to FIG. 2 d and neednot be further elaborated. The surface 68 forms part of a drive member70 which contacts the displacement member 32 (see FIGS. 3 a, 3 b) inorder to drive the y displacement, so a displacement in a directionperpendicular to the plane of drawing. The contour of the surface 68 isshown in the front face of the displacement member 32, that is, in acentral position. The displacement member 32 can be displaced in the xdirection as well as in the y direction; this motion should not beimpeded by the drive member for the y direction. This desired effect isachieved in that the contact between the front face of the displacementmember 32 and the drive member 70 is realized via the fluid bearing.

FIG. 3 consists of the FIGS. 3 a and 3 b. FIG. 3 a is a firstperspective view of the apparatus in accordance with the invention. FIG.3 b is a second perspective view of the apparatus in accordance with theinvention, in which a number of elements has been omitted (that is, incomparison with FIG. 3 a) in order to afford a better view of the laserdistance sensor. FIG. 3 shows the drive member 70 for driving the ydisplacement of the displacement member 32. Also shown is thedisplacement stage 48 which is formed by the combination of thedisplacement guides 24 and 26 and the displacement member 32. Thisassembly is arranged on a rigid supporting frame 72. The system forlaser distance measurement also forms part of the apparatus shown. Thissystem comprises the three mirrors already described with reference toFIG. 2 a, each of said mirrors being situated in a respectiveco-ordinate plane on the outside of the object carrier 35. With each ofthe mirrors there is associated a respective source of laser light 74 a,74 b and 74 c. These sources may be separate lasers, but may also beformed by a single laser wherefrom the laser light is conducted to eachof the mirrors, for example, via optical fibers. With the aid of suchlaser sources and mirrors there is formed a laser distance sensor whichoperates on the basis of the known interference principle for accuratedetermination of the position of the object carrier. The signal derivedfrom the laser distance sensor is applied to the computer 54 whichcontrols the drive means on the basis of this signal in such a mannerthat the drive member reaches the desired position.

1. An apparatus for the measurement or machining of an object (37),which apparatus is provided with a displacement stage (48) for thedisplacement in at least one co-ordinate direction of a part of theapparatus (32, 34) which is to be displaced relative to a frame (2) ofthe apparatus, which displacement stage includes: two supportingwedge-shaped displacement guides (24, 26) which bear on a supportingsurface (20) of the apparatus and each of which is provided with a wedgeface (28, 30), said guides being arranged opposite one another in such amanner that their wedge faces face one another, a displacement member(32) which is connected to the part of the apparatus to be displaced andis provided with two wedge faces (34, 36) which co-operate with thewedge faces (28, 30) of the displacement guides, the displacement memberbeing supported by the wedge faces of the displacement guides as aresult of said co-operation, which apparatus is also provided with drivemeans for the displacement of the two displacement guides in such amanner that the distance between their wedge faces varies while theorientation of the wedge faces relative to one another remains the same,characterized in that the apparatus is provided with control means forcontrolling the drive means, and that the drive means and the controlmeans are arranged for mutually independent displacement of the twodisplacement guides.
 2. An apparatus as claimed in claim 1, in which thewedge faces of the displacement member bear on the wedge faces of thedisplacement guides by way of fluid bearings.
 3. An apparatus as claimedin claim 1, in which the displacement guides bear on the supportingsurface of the apparatus by way of fluid bearings.
 4. An apparatus asclaimed in claim 1, in which the displacement guides (24, 26) aredisplaceable across the supporting surface (20) of the apparatus alonglinear guides (56) which are provided on the supporting surface, thedisplacement guides bearing on the linear guides by way of fluidbearings.
 5. An apparatus as claimed in claim 1, in which thedisplacement member (32) is displaceable, relative to the displacementguides (24, 26), in the direction of the line of intersection of itswedge faces, the apparatus is provided with a drive member (70) whichcontacts the displacement member in order to drive said displacement,and the contact between the displacement member (32) and the drivemember (70) is realized by way of a fluid bearing.
 6. An apparatus asclaimed in claim 1, in which the part of the apparatus to be displaced(32, 34) is provided with at least one mirror surface (38) and in whichthe displacement in at least one co-ordinate direction is measured bymeans of a laser distance sensor (74, 38, 40) which comprises saidmirror surface.
 7. A displacement stage for an apparatus for themeasurement or machining of an object comprising: two supportingwedge-shaped displacement guides (24, 26) which bear on a supportingsurface (20) of the apparatus and each of which is provided with a wedgeface (28, 30), said guides being arranged opposite one another in such amanner that their wedge faces face one another, a displacement member(32) which is connected to the part of the apparatus to be displaced andis provided with two wedge faces (34, 36) which co-operate with thewedge faces (28, 30) of the displacement guides, the displacement memberbeing supported by the wedge faces of the displacement guides as aresult of said co-operation, the apparatus being provided with drivemeans for the displacement of the two displacement guides in such amanner that the distance between their wedge faces varies while theorientation of the wedge faces relative to one another remains the same,the apparatus being provided with control means for controlling thedrive means, and the drive means and the control means being arrangedfor mutually independent displacement of the two displacement guides.